Liquid gauging is the determination or measurement of a quantity of liquid in a container. The quantity of liquid in a container can be defined or expressed in different units of measure, such as volume, weight, mass and height of the liquid surface in the container. As an example, in aircraft applications, aircraft fuel is contained in a plurality of fuel tanks on the aircraft. Typically, it is the mass of fuel in a tank that determines in large measure the flight distance for the aircraft. Although a measure of the fuel volume and weight may also be useful, ultimately it is an accurate determination of the mass of fuel in the tanks that is of primary interest.
The measurement of fuel mass, or any liquid mass in a container, is a complicated process when a high accuracy is required. First and foremost is the observation that no sensor can directly measure mass of a liquid. The liquid mass must be calculated based on a measurement of parameters that are related to mass, such as by measuring the liquid volume and then either measuring or assuming a density value for the liquid. In aircraft fuel gauging, the process is complicated by numerous factors, including aircraft dynamics maneuvers, acceleration effects, and fuel density changes due to temperature, different fuel blends, pressure variations from altitude and atmospheric changes, to name a few of the more important variables. Aircraft fuel gauging is necessarily a dynamic process during flight. The problems associated with fuel gauging are exacerbated by the fact that most fuel tanks, particularly those in larger aircraft, whether the aircraft be commercial or military, can have very complicated geometric shapes and further typically include a number of interior structures.
Known liquid gauging systems typically gauge fuel by attempting to determine the height and orientation of the liquid surface in the container. Once the liquid surface plane is defined and located, the information can be converted to a volume and then mass. Such systems typically use ultrasonic sensors, or liquid height sensors such as elongated capacitance sensors, to determine height of the liquid in the container. Another known approach is to use pressure measurements to determine the location of the liquid surface plane in combination with a densitometer and acceleration.
All of these known approaches are fundamentally dependent on calculated volume and density values, which calculations are based on the output signals from various pressure, temperature and height-based sensors. Unfortunately, all sensors are inherently inaccurate to one degree or another, so that for the known systems, overall system accuracy is determined by the collective accuracy (or conversely, inaccuracy) of all the sensors. For dynamic systems such as are used in aircraft fuel gauging, not only are there these sensor inaccuracies but also there are all the dynamic parameters such as aircraft movement and environmental changes that often contribute an indeterminate amount of system level uncertainty into the measurements and the calculations based on those measurements. Yet a further level of complexity arises from the consideration that sensor accuracy can change over time, and sensors can fail. When these events occur, conventional fuel gauging systems can suffer a significant degradation in performance.
Merely adding more sensors for redundancy in conventional systems may provide some increase in reliability and fault tolerance, but the final fuel mass/volume determination will still be limited by the collective accuracy of the various sensors and system uncertainties. More accurate sensors are often developed, but usually at a significantly increased cost. Known gauging systems attempt to reduce the effects of sensor inaccuracy and system related uncertainties by developing or providing various compensation schemes. For example, density and pressure determinations may be compensated for temperature variations and fuel stratification within a tank. Or height calculations may be compensated for internal tank structures, tank geometry and acceleration effects during flight. Such attempts to compensate sensor data based on fluid dynamics and system uncertainties can be helpful but still tend to be limited in the extent to which such compensation can accommodate dynamic changes.
The objectives exist, therefore, for a wholly different approach to liquid gauging that significantly increases system accuracy while still using conventional system parameter measurements.